The invention relates generally to a gasification system, and more particularly to a multifuel, multizone gasification system.
Due to increasing crude prices, and relatively long term availability of coal, coal is being looked at as a one of the primary fuels of the future. Gasification of coal is an option that is environmentally more benign as compared to other methods of generating energy from coal, such as pulverized coal combustion. Coal gasification systems can be designed to remove CO2 before combustion of syngas produced as a result of gasification. Sequestration of CO2, however, is a technology currently being developed and has not yet been demonstrated on an industrial scale.
Gasification of biomass is another energy generation option. It uses renewable feedstock—biomass. Some examples of biomass include wood and other plant derived feedstocks. Plants consume carbon dioxide from environment during their growth. If biomass is utilized in gasification, the amount of CO2 released in the environment due to gasification, corresponds to the amount of CO2 consumed during growth of plants. Thus gasification or combustion of plant biomass does not add extra CO2 to the environment. Therefore, use of biomass is considered carbon neutral. The plant biomass can be grown relatively quickly as compared to other carbonaceous feedstocks. Utilization of biomass feedstocks helps reduce dependence on fossil fuel since they are renewable and can be grown relatively quickly. Thus, the use of biomass for power generation is attractive from the perspective of sustainability and environmental impact.
However, biomass contains a large amount of oxygen and moisture as compared to coal. The ash content can also be significantly higher; the exact quantity of ash depends on the source of biomass employed. Due to these aforementioned components, that do not have any heating value in themselves, the calorific value of biomass is much lower than that of coal. Due to lower heat content, gasification of biomass on scales comparable to coal combustion, such as toward production of 100s of MW of electricity, needs a large amount of biomass, of the order of hundred thousand tons per year. It becomes difficult to ensure a steady supply of such large quantities of biomass. Thus large systems based purely on biomass as a feedstock, tend to become practically unfeasible due to supply chain issues in procuring large quantities of feedstock. Hence, dedicated biomass conversion facilities such as biomass-fired boilers or gasifiers are typically limited in sizes up to 25 MW thermal equivalent.
Gasification systems using both coal and biomass have obvious advantages. Such systems have been proposed and developed earlier, but these have faced numerous operational issues. The simplest co-gasification systems use existing coal gasification systems and add biomass to coal. A few technology developers attempted to develop a system for biomass co-injection with coal, but such systems are typically limited to less than 5-10% of biomass by energy content. Such co-injection approaches cannot be readily extended to larger amounts of biomass for example due to layer formation in the feeding and storage systems resulting from different densities, sizes, and shapes of coal and biomass particles.
These systems face multiple problems, not only with the gasifier, but also with auxiliary systems such as the feed handling systems. For example, many types of biomass have a fibrous character that makes it difficult to grind as compared to relatively brittle coal. A common size reduction setup cannot handle a mixture of coal and biomass, or cannot produce satisfactory results.
It is desirable to have a gasification system capable of gasifying both coal and biomass, without having to face the aforesaid problems. It is also desirable to develop a method of operating gasifier that can gasify multiple fuels without operational problems. It is also desirable to have a method that can retrofit existing coal gasifiers to achieve fuel flexibility.